Women who carry cancer-predisposing germ-line mutations in BRCA1 have approximately 80% chance of developing breast cancer in their lifetime. In addition, 30-40% of sporadic breast cancer cases display reduced BRCA1 expression, suggesting that BRCA1 dysfunction may be associated with both familial and sporadic forms of breast cancer. While loss of the DNA repair function of BRCA1 most likely contributes to breast cancer development, such a defect alone may not be sufficient to explain the tissue-specificity of BRCA1-associated tumors. Thus, the exact molecular basis for the tissue-specific tumor suppressor function of BRCA1 remains an unresolved issue. BRCA1 has been reported to bind to estrogen receptor alpha (ERalpha) and modulate its transcriptional activity. Reinforcing the functional link between BRCA1 and ERalpha, we have recently identified a novel BRCA1-interacting protein (COBRA1) that co-regulates a common set of estrogen-responsive genes with BRCA1 in breast carcinoma cell lines. Perhaps more intriguingly, we have also uncovered a new function of BRCA1 in modulation of estrogen biosynthesis in adipose stromal cells (ASCs). These findings point to a need to look outside the "box" of epithelial cells and to elucidate BRCA1 functions in the context of the mammary microenvironment. We therefore hypothesize that BRCA1 blunts estrogen actions by repressing ERalpha-mediated gene expression in breast epithelial cells and reducing estrogen production in adipose stromal cells. BRCA1 in multiple cell types may act cooperatively to suppress mammary tumorigenesis. We propose three Specific Aims to test this central hypothesis. Aims 1 and 2 will investigate the impacts of BRCA1 on estrogen-mediated gene expression in breast cancer cells and estrogen biosynthesis in adipose stromal cells (ASCs). Aim 3 attempts to integrate the studies of epithelial and stromal cells by utilizing a cell co-culture system and tissue-specific Brca1 knockout mice to study BRCA1 function in the context of the tissue/tumor microenvironment. Relevance: The notion that BRCA1 may function in multiple cell types within the mammary microenvironment represents a conceptual advance in BRCA1 research. Findings from these studies promise to offer important guidance to the development of more targeted and effective strategies for the prevention and treatment of BRCA1-associated cancers.